Introduction

Approximately 7.2 billion humans inhabited the Earth in year 2013. By comparison, there might be 500,000 elephants of different kinds, 200,000 chimpanzees, 100,000 gorillas, 20,000 polar bears, 3,000 tigers, 2,000 giant pandas and 200 California condors. Notably, the human population has grown about ten-fold over the past 300 years and nearly four-fold in just the last century. This monumental historical development has profoundly changed the relationship of our species to its natural support systems and has greatly intensified our environmental impact, particularly regarding species extinctions. Equally amazing are the signs that, in our generation, the human population explosion is abating (Figure 1; note that, here and below, many of the values given are estimates and, after the year 2005, projections). Our numbers are expected to rise by another 50% before reaching a peak some demographers expect late in this century; many researchers posit that a decline is likely to follow. What caused this population surge; where are we headed; and how might the proliferation of our species affect its future well-being?

Some current demographic trends

Until modern times, the growth of our numbers was slow and variable. A pronounced expansion began with the advent of the Industrial Revolution, about two centuries ago. Whereas tens of thousands of years passed before our species reached the one billion mark, around 1800 AD, it took only 123, 33, 14, 13, 12 and 13 years to add each succeeding billion. This accelerating rate of increase is what is meant by the term population explosion. Around year 1963, population growth reached a maximal rate of about 2.2% per year—perhaps a thousand times faster than expansion in prehistoric times. The annual increment has since dropped by half–to 1.1% per year (or, as demographers prefer, to 11 per thousand per year)–and it is still decreasing. Nevertheless, a cohort of about 80 million individuals, about the population of Germany, is being added to the planet each year (Figure 1).

Figure 1. Long-term world population growth, 1750-2050. Source: United Nations Population Division

Figure 2 shows that fertility is now declining with time. Worldwide, the average woman currently bears 2.5 live offspring. In several African nations, fertility still exceeds seven live births per female. At the other extreme, the average woman in the Pacific rim nations of East Asia and in much of Europe bears approximately 1.3 live babies. Correspondingly, population growth rates vary with locale, from more than 3% per year in some African nations to a slightly negative rate (i.e., declining population) in some Eastern European states. The U.S. has the highest rate of procreation and of immigration—each roughly 0.5% per year–giving it the greatest overall population growth rate of any industrialized nation.

Figure 2. Fertility trajectories of the world and major development groups, 1950-2050 (medium variant)Source: United Nations Population Division, World Population Prospects: The 2004 Revision. Highlights

The average human life-span has risen from 30-40 years in pre-industrial times to about 70 years today (Figure 3). Longevity is still about 50 years in Angola while it exceeds 80 years in northern Europe and Japan. In developing nations, longevity has sometimes increased by more than half a year in a calendar year! At the same time, life-span has been adversely affected in parts of Africa by infectious diseases such as HIV-AIDS and, following the end of communist rule in Eastern Europe, by alcohol consumption.

Figure 3. Life expectancy at birth for the world and major development groups, 1950-2050. (Source: United Nations Population Division, World Population Prospects: The 2004 Revision. Highlights)

The demographic transition

Demographers have a good understanding of why the growth of the human population exploded and then, rather abruptly, declined toward its long-term value. The math is simple: excluding migration, the rate of change of the number of individuals in a population is simply the difference between their birth rate and death rate. The explosion in human population thus reflects the excess of births over deaths brought about by the Industrial Revolution. Until about two centuries ago, birth rates and death rates were both high. Because these two rates were about equal in magnitude, the population grew slowly, albeit unevenly. For example, human numbers grew at roughly 0.25% per year in 1700 C.E. (This amounts to a doubling of the population about every 300 years.) Soon thereafter, as discussed below, institutional and technical advances caused death rates to fall in one nation after another around the globe. But because birth rates remained high, population growth rates soared, an unintended consequence of the alleviation of human hardship in the modern era. Then, after a few decades of declining death rates, the families in those nations developed the inclination and found the means to dramatically limit procreation. As a result, fertility rates fell, often rapidly, to approach the low death rates, and population growth slowed.

Figure 4. The classic stages of the demographic transition. Population Bulletin, Transitions in World Population)

A theoretical model called the demographic transition divides this general pattern of population growth into four idealized stages, as illustrated in Figure 4. In early times, birth rates and death rates are high (perhaps each near 5% per year) but similar. There is then little sustained growth in the population. In the second stage, death rates decline but birth rates remain high; consequently, net population growth increases, as indicated in the figure by the shaded balloon labeled natural increase. Third, birth rates decline and approach the low death rate, causing population growth to subside. At that point, birth and death rates become similar again (perhaps each at 1% per year) and the net growth rate approaches zero or even becomes negative. Stage 4 is not like the pre-transition state (Stage 1), since the size of the population will have expanded and longevity increased during the demographic transition.

The demographic transition paradigm can be applied both to individual nations and also to the world population as a whole. The historical prototype is 19th century England during its industrialization. Soon thereafter, the transition followed industrialization to Western Europe and the United States. As they modernized in the 20th century, Japan (followed by a succession of other Asian nations) replicated this transformation. The decades following World War II and the end of the colonial era saw most developing nations embark on this path; now, their death rates are typically at low levels and their birth rates are on the way down. Thus, most nations are currently somewhere in the third stage of their demographic transition and some are in stage four. Consequently, the growth rate of the world population in year 2011 was 1.1%–the difference between a birth rate of 1.9% and a death rate of 0.8% per year.

The demographic transition has stalled midstream in some developing nations where customs and traditions remain strong. That is, low death rates (say, 1.5% per year or less) have been achieved but birth rates linger at 4 to 5% per year. So it is that population growth in Niger, Mali and Uganda exceed 3% per year, corresponding to a population doubling time of less than 25 years. (Doubling time in years is approximately equal to 70%/growth rate in percent per year. In this example, the doubling time would be 70/3 or 23 years.) This situation can lead to a demographic trap where rapid population growth undercuts the very technical, social and economic progress that could otherwise resolve it. Developing nations as a group now have 80% of the world’s people and contribute 97% of population growth. The on-going increase of the human population can therefore be understood to represent unfinished demographic transitions in diverse less industrialized societies.

Why death rates have declined

Infectious agents has always been a major cause of human mortality. Particularly important diseases are malaria, tuberculosis, measles, influenza, cholera, and a variety of parasitic infections. (In recent years, HIV/AIDS has overshadowed many of these.) In addition, diarrhea and respiratory diseases of bacterial and viral origin ravage the children in poor nations. Infant mortality can amount to more than 10% of the live births in those settings, more than 20-times that in many industrialized states. The battle against infectious diseases gained force early in the modern era through the development of public health regimes. Thus, long before the era of twentieth-century patient-directed medicine, we learned how to avoid the perils of contaminated drinking water, to drain swamps where mosquitoes harbor the malaria parasite, to immunize the young, to quarantine the infected and to teach public sanitation and private hygiene. (A classical example of an early public health intervention was the introduction of vaccination against smallpox by Edward Jenner more than two centuries ago; this scourge has now been entirely eradicated.) In addition, improved nutrition not only saved lives by itself but also strengthened resistance to infection. These simple preventive strategies were often inexpensive, and colonists brought them along to protect themselves and their workers in the far-off lands. Today, the transfer of technology and know-how from more developed countries (MDCs) continues to play an important role in alleviating illness and death in many less developed countries (LDCs). Especially important is the steadily decreasing mortality rate in infants and children. At the same time, global modernization has brought a new demographic trend to the developing world: cardiovascular disease and cancer now rival or surpass infectious diseases as health hazards.

Why birth rates have declined

Children are naturally loved and valued for themselves. But, especially in traditional (typically, agrarian) settings, children are also seen by their parents to be economic assets: a ready source of capital and security when alternatives are out of reach. Sons are particularly valued, since it is they who do the heavy labor, inherit the family plot and take care of aging parents. For practical reasons, daughters are often less desired: they may be regarded as less productive and as likely to marry and move on, often with a costly dowry payment. Thus, time-honored wisdom might suggest an investment strategy of having, say, eight offspring. A parent would then expect four sons, one or two of whom will hopefully survive to adulthood, worked the farm and, in time, care for the parents. Such schemes of providing security to the family become institutionalized in cultural norms and shared practices.

Under the most favorable conditions, a woman can bear as many as 20 children in her lifetime. But this is rare. Parents universally chose to limit family size because too many children impose costs in excess of benefits. Thus, a variety of practices have long been used to thwart pregnancy; for example, celibacy, late marriage, sexual abstinence and the rhythm method. Of particular importance in traditional societies, however, is prolongation of the nursing of children. (This is because lactation inhibits ovulation through the mother’s endocrine system, thereby reducing the risk of pregnancy.) In addition, desperate measures to control family size are frequently taken by those who lacking good options. For example, perhaps 20 million pregnancies a year, more than one-tenth of the total, are ended by unsafe (septic and/or criminal) abortion despite serious risks to the mothers. Infanticide is another long-standing expedient. Especially in hard times, baby girls are selected against. For example, ultrasound previews, while typically illegal, are widely used in Asia these days to pick out female fetuses for abortion. The various practices favoring male heirs is said to account for “100 million missing women” world-wide.

Just as the Industrial revolution precipitated a fall in death rates, it has also driven the subsequent fall in birth rates and, consequently, a resolution of the population explosion. This is because modern and modernizing societies have provided alternatives to the creation of large families in pursuit of security and economic advancement. This is not just a matter of wealth. Rather, modern countries have elaborated civil institutions that provide a social safety net. The safety net promotes health and education; property rights (ownership) and civil rights (e.g., the vote, equality before the law, public safety and women's rights); some measure of financial security (e.g., insurance, loans, retirement plans, unemployment benefits, job creation and retraining programs); and income redistribution (e.g., public relief and welfare programs as well as graduated taxation). Individual aspirations then become reoriented from creating family-based security to self-realization, and from subsistence to advancement. This modern agenda enables even poor families in traditional settings to have fewer children and to invest more in each.

Women in MDCs typically anticipate having two children and, these days, even fewer. Their prospects and security are assumed, allowing them to marry late—or not at all. Furthermore, contraception is widely available and the choices are diverse; notably, condoms, cervical loops and caps, vaginal spermicides and surgical sterilizations for both There are also pills to prevent ovulation (i.e., oral contraceptives), to induce the early miscarriage of a pregnancy and to thwart the implantation of a fertilized egg “the morning after.” Surgical abortions are common and (quite the opposite of criminal or septic abortion) safer than live births.

Individuals in impoverished nations are steadily becoming aware of modern lifestyles through schooling, trade, migration and mass media such as radio or television. Indeed, nearly half of the human population—a steadily rising fraction—now lives in cities from which they report trends to those back home. Cultural diffusion has also alerted pre-modern communities (and their national governments) to the socio-economic burden of high fertility. Their exposure to the developed world has also shown them ways to build social welfare and to limit procreation in advance of industrialization and wealth creation (see Kerala). Foremost among these countervailing forces is a couple’s anticipation of improved health and longevity for the members of the family, young and old. Similarly, a sense of security increases among those in poverty with opportunities for their education, physical and social mobility, economic advancement and accumulation of modest personal savings. Food security in impoverished communities, while often problematical, has nevertheless risen steadily in recent decades just as infant mortality has fallen. Transfer of low cost know-how and materials from developed nations has also greatly benefited those less industrialized. Confidence in one’s government helps too. All of these factors have encouraged individuals in LDC to address poverty through personal and social measures that, among other effects, supplant the information to large families with personal empowerment. A result is declining population growth all over the world (Figure 5). And economic development can then follow.

Figure 5. Average annual rate of change of the population of the world and major development groups,1950-2050 (medium variant) Source: United Nations Population Division,World Population Prospects: The 2004 Revision. Highlights

The empowerment of women (e.g., through schooling and employment opportunities) has been of special importance, as have international efforts to provide access to family planning assistance and contraception where needed. The steady decline of LDC fertility and population growth reflects the impact of these processes (Figures 2 and 5). Nevertheless, perhaps 200 million women in LDC presently express the unfulfilled desire for fewer children and for family planning assistance such as information and contraceptive devices. On the other hand, many traditional mothers still aspire to large families (Figure 6). Thus, the population explosion is far from over.

Human population prospects in the twenty-first century

Figure 6. Percent of married women of reproductive age with two living children who do not want another child. (Source: Population Reference Bureau, 2006 World Population Data Sheet)

The overall human death rate is not likely to change significantly in the foreseeable future. It will presumably decline in nations like Russia which have recently suffered sociopolitical upheavals. The opposite trend can be expected for developing nations as their youthful populations age. Globally, birth rates will probably continue to decline in the coming decades since, nowadays, couples are increasingly prone to limit their family size, whatever their wealth. Coercion by national governments, such as China’s one-child policy, appears to be unnecessary. If and when the global birth rate again matches death rate, we will hit zero population growth. This could occur by the year 2070 when the population might be 9.5 or 10 billion. Negative population growth may then ensue, as is now approaching in an increasing number of MDCs.

Although this is an era of decreasing birth rates, diverse constituencies nevertheless regard procreation as either a good in itself or as a practical imperative. Among these pro-natal voices are the following:

Standard economic thinking holds that human capital (e.g., labor and intellectual creativity) drives wealth creation; hence, the more people, the better off we are. Such a cornucopian view foresees no limits to an indefinitely expanding economy, environmental impacts notwithstanding. At the same time, many economists anticipate a point beyond which further additions to our numbers will diminish our well-being.

A large fraction of MDC governments would like the demographic profile of their nation to be youthful. Their premise is that the young provide the nation’s workforce and army with vitality and strength. In addition, young wage-earners are a source of tax revenues.

Those concerned with the well-being of the elderly see that an increasing dependency ratio (here, retirees per worker) will increasingly burden national retirement plans, such as U.S. Social Security. (The governments of some LDCs are grappling with the opposite issue: high population growth can undercut their social welfare and economic progress.)

Various cultural and religious norms encourage procreation. In some communities, men are expected to show virility by siring a large brood. It is sometimes a mother-in-law’s prerogative to press for more grandchildren.

Employers may welcome a large labor pool in the hope that competition for jobs will drive down workers’ wages.

Many in poverty still see their progeny as their best hope in desperate times.

With increasing affluence, many parents can afford to value children for their intrinsic worth and enjoy large families.

For these various reasons most MDC governments favor pronatal policies, and many now maintain programs that encourage procreation. Furthermore, the level of infant and child mortality in some LDCs is still so high that parents feel the need for larger families. Nevertheless, the trend toward decreasing family size continues around the globe, impelled in good measure by growing opportunities for women outside of the home. This unintended consequence of modernization and globalization is welcomed by those who, like Malthus early in the Industrial Revolution, view overpopulation as a fundamental threat to human welfare. From the field of ecology, with its concern for carrying capacity and overshoot, comes the question of whether humans might exceed the limits to growth in a finite habitat. One way this issue has been analyzed is the calculation of the ecological footprint; i.e., that portion of the ecosphere usurped by the activities of an individual, a group or a nation. Applications of this admittedly imprecise parameter have raised the possibility that our current practices exceed the bounty of this planet and are therefore unsustainable (see sustainable development). Another perspective posits that human impact increases with 1) our numbers; 2) material consumption per person (see Consumption and well-being); and 3) the extent of environmental damage caused by each unit of that consumption. (See the IPAT equation.) Since it seems unfair for the rich to thwart the material progress of the poor, politically difficult to reign in the appetites of the affluent and technically challenging to mitigate the impact of their consumption, curtailing population size offers a practical approach to mitigating human impact. While the Earth’s carrying capacity for our species is not known, the precautionary principle advises that it is better to be safe than sorry.

Demeny, P. 1990. Population. In B.L. Turner (Editor), The Earth as Transformed by Human Action: Global and Regional Changes in the Biosphere over the Past 300 Years. Cambridge University Press with Clark University, Cambridge; New York, 41-54. ISBN: 0521446309